I was surprised that anyone, let alone “Today’s Apple”, would kill the iPhone SE so quickly. It was introduced in March 2016 and got a storage bump in March 2017. In the world of lesser-loved and neglected Apple products, it was practically brand new. Apple is all about keeping products around forever: the Mac mini from 2014, the Mac Pro from 2013, the iPad Mini from 2015, the non-retina MacBook Air from 2015. 3-plus years without an update or price drop is common now. To kill it with no warning just before the new 2018 iPhones came out was practically unprecedented.
Update, October 23, 2020: Hooray! I was afraid that small iPhones were gone forever, but today Apple came out with the iPhone 12 Mini which is just what I was hoping for/expecting: an X-style device (edge-to-edge screen, no home button) that's about the size of an old 5S/SE. (Photo 1, photo 2.) I hate that the power button has moved to the side, and I'll miss the headphone jack some, but overall it looks good.
Wikipedia says “Apple stated in the ‘Let us loop you in’ event on March 21, 2016 that they sold over 30 million 4-inch iPhones in 2015” , so either a) that number quickly dropped to almost zero in the intervening 3 years, b) they don't want to support the A9 CPU that is shared by the 6S, which they also stopped selling the same day as the SE, or c) they don't want to support a 320-point-wide screen.
Option (a) has never stopped Apple from selling a product past its expiration date before. (See the iPod shuffle and the iOS-looking iPod nano.) Option (b) would give me hope were it not for the fact that the just-released iPad Mini 4 still has an A8. So I can only conclude that option (c) is it, and Apple has given up on devices with a screen smaller than 375 points. RIP one-handed iPhones. :-(
The timing of the discontinuation also means that the iPhone SE and 6-series were never sold new with iOS 12, which might free Apple from a trap similar to the one they were in when they were still selling the ancient 3GS with iOS 6. I bet that the next version of iOS that drops support for old hardware (probably not 13, likely 14, definitely by 15) will leave the SE and 6-series behind, though I still don’t see that as being strong enough pressure to kill the SE so suddenly. Some iPhones have been supported by more versions of iOS than others.
I love iPhones. I got my first shortly after it was released in 2007. (Very soon after. Just before the price drop, in fact, but close enough to it (within 14 days) that I got a full refund on the difference via Apple's standard policy.) I've had an original iPhone (2G), a 3G, a 3GS, a 4, a 4S, a 5S, and an SE. I only skipped the 5, and I never went to the 6 or anything with a larger screen. I'm around computers all the time. I'm seldom far from one. I do not need a gigantic phone, because as soon as I have anything non-trivial to do, I turn to a computer to do it. I prefer my phone to be physically small, and I like being able to reach all of the screen comfortably with one hand. I also like other things about it, like the square sides and the headphone jack. The best I can hope for at this point will be a XR-style device roughly the size of an SE with a 375×667 point screen the size of an iPhone 6/7/8. That would be physically an OK size, though not quite usable by a single hand. Given the choice between half of what I want or none of what I want, I’ll take half. But I’m worried there will be no choice at all.